The present invention relates to a method of preventing "compromising radiation" during the processing and transmission of secret data information.
Secret information is protected against unauthorized access during transmission by means of cryptographic processes. These processes have recently been developed mathematically and technologically to such an extent that they are considered to be "unbreakable," and the security against decyphering of such processes has become calculatable. It is no longer possible for the listener to coded information to decypher the information without knowing the code material. However, the listener can use other methods of gaining information: he can evaluate the optical, acoustical, electrical or electromagnetic radiation of the devices in a system which processes codes and transmits information. Therefore, high expenditures are required to install processing centers so that they are located in shielded areas and are secure against listening-in. Data processing centers are in communication with the environment via various "channels" and each "channel" can be tapped by a listener outside the protected area. The term "channel" is here understood in the communications technology sense and a distinction is made between data channels in which information is intentionally transported, e.g., telephone lines or radio paths, and channels in which information is transported inadvertently, e.g., public electricity lines, water mains, windows, etc.
Data information is usually transmitted intentionally and unintentionally in the form of electrical signals. (An exception is the optical transmission of data). In the case of electrical signals, the signal is transmitted intentionally and unintentionally in its original signal form and also in a modulated form, e.g., AM, FM, etc. If the signal is modulated onto a high frequency carrier, it propagates through space as well as over all electrical lines. If the original signal is secret data information, the undesirable propagation and transmission is called "compromising radiation."
Since the "compromising radiation" may undermine the efficiency of cryptologic equipment, encrypting devices are checked with particular care for "compromising radiation" and much engineering effort is required to prevent such "compromising radiation."